Atomic Theory: Spectroscopy and Flame Tests Introduction Light energy is also known as electromagnetic (EM) radiation. The light that we observe with our eyes, visible light, is just a small portion of the electromagnetic (EM) spectrum. Other forms of light energy or EM radiation include gamma rays, x-rays, ultraviolet rays, infrared radiation, microwaves, and radio waves. These different forms of EM radiation possess different energies, as well as wavelengths and frequencies, but all travel at the same speed, the speed of light. EM radiation has wave-like properties and are thus characterized by their wavelength and frequency. The image to the right shows two l A crest different waves. The wavelength, symbolized by the Greek letter lambda ( ), is the distance between two wave crests or peaks, which is equal to the distance trough between two troughs. Notice that the wavelength for l B the top wave, indicated by A, is greater than the wavelength for the bottom wave, B. Since energy waves move, we can count the number of crests or peaks that pass a given point in one second. This is called the wave s frequency, which is symbolized by the Greek letter nu (v) and measured in units of cycles per second called Hertz (Hz). Remember that all EM radiation travels at the same speed, so the smaller the wavelength (the distance between two crests), the higher the frequency. Frequency and wavelength are related according to the following equation: c = v, where c is the speed of light and is exactly equal to 2.99792458 10 8 m/s. The energy of the EM radiation is directly related to its frequency, according to the following equation: E = hv where h is Planck's constant, and is equal to 6.626 x 10-34 J s. When all the electrons in an atom are as close to the nucleus as possible (in the energy lowest orbitals) the atom is most stable. This is called the ground state electron configuration. When an atom absorbs energy, its electrons become excited and will move from lower energy orbitals (the ground state) to higher energy orbitals. Since the electrons prefer to be closer to the nucleus, they quickly return to lower energy orbitals. When this happens, EM radiation that equal in energy to the energy difference between the two states is emitted. If this energy is in the visible range of the EM spectrum, specific colors are observed. Since different types of atoms have different numbers of protons and electrons, they emit light at different wavelengths, and thus each type of atom has characteristic emission spectrum. When an atom is heated it emits a characteristic color. If this light is viewed using diffraction grating, we observe of a series of colored lines called a line spectrum. In this lab, you will investigate the visible light emitted from various atoms by performing flame tests and observing line spectra. The wavelength, frequency, and/or energy of various forms of electromagnetic radiation will also be calculated. GCC CHM 151LL: Atomic Theory GCC, 2017 page 1 of 8
For more information: Chemistry: Atom s First by OpenStax sections 3.1 Electromagnetic Energy and 3.2 The Bohr Model Equations to use for the calculations: c = v c represents the speed of light, 2.998 x10 8 ms -1 wavelength in meters frequency in s -1 E = hv h=6.626 x 10-34 J s 10 9 nm = 1 m 1 s -1 = 1 Hz Materials: 7 test tubes test tube rack 150 ml beaker solutions of LiNO3, Cu(NO3)2, KNO3, Sr(NO3)2, NaNO3, Ba(NO3)2 Bunsen burner stryker wood splints DI water bottle Genesys Spec-20 Spectrometer pre-cut filter paper strips colored pencils calculator Procedure PART I. Spectrophotometric Analysis Of Light 1. Turn on the Spec-20 Genesys spectrophotometer, using the switch on the back side of the instrument at the start of lab; it will take about 15 minutes to warm up before use. 2. On the top right of the instrument is a sample holder that holds the sample cells, called cuvettes. Never insert chemicals directly into the Spec-20, or you will contaminate and damage it. To the left of the sample holder is a panel of buttons that control the wavelength (in nanometers) and output (percent transmittance or absorbance) settings. The up/down arrows by wavelength turn the prism-like device in the Spec-20, so light of only one wavelength strikes the sample in the cuvette. 3. Use the up/down arrows to adjust the wavelength to 400 nm. 4. Place a pre-cut strip of filter paper in the sample holder in a diagonal fashion. This is to reflect the light beam within the spectrophotometer. Stand where each student can see the side of the filter paper that faces the light source. You should see a tiny dot of color on the paper. The color observed is the color of light with a wavelength of 400 nm. 5. Use the up/down arrows to scroll through wavelength values from 400 nm to 750 nm in 25 nm increments to find the colors corresponding to the different wavelengths of the visible spectrum. 6. Use colored pencils to color in the spectrum in the box on your report sheet to show the correlation between color and wavelength. Note: There will be many more shades of color than the GCC CHM 151LL: Atomic Theory GCC, 2017 page 2 of 8
seven used in the often-used pneumonic device ROYGBIV (which means: Red, Orange, Yellow, Green, Blue, Indigo, and Violet). Therefore, you may have to use some creative color blending of colors. PART II. Gas Discharge Demonstration 1. Your instructor will show samples of gas collected in thin glass tubes known as gas discharge tubes. The ends of the tubes have electrodes that allow a current to pass through the gas and light up. When inserted into a voltage source, the samples will glow a characteristic color. When the light is diffracted (e.g., with a prism or the diffraction glasses we will be using in class), you can see the separate spectral lines that make up each sample. 2. In the 2 boxes at the top of your Lab Report, draw color representations of the diffracted light from the two gas samples your instructor shows. PART III. Flame Tests 1. Label your 6 test tubes with a pencil one for each solution listed above. From the reagent station, put about 10 12 drops of each solution into the appropriate test tube. Place one splint into each test tube an soak for about 5 10 minutes in order to absorb enough solution. 2. Obtain one unknown from your instructor per group. Place 10-12 drops of the unknown solution in a clean test tube and place a wood splint in the test tube of the unknown. Allow sufficient time to soak. Be sure to record your unknown number in your lab notebook. 3. Prepare your 150 ml beaker with about 20 40 ml of water to dispose of the used splints. 4. Light your Bunsen burner with a striker. See technique Using a Bunsen burner. 5. Grasp the LiNO3 wood splint by the tip and place the damp end of the splint in the middle of the flame (in the tip of the inner blue cone) for a short time (about 2 3 seconds). You should see the color of the metal ion burning in the first few seconds. Avoid burning the wood splint itself. A wet splint cannot burn. If you start to notice the splint burning, it was in the flame past the point of dryness. If it does start to burn, you should immediately dip the splint back into the correct test tube to put out the flame, re-wet the splint and test the flame color again. You can repeat this several times if you have difficulty seeing the color. 7. When you are done testing a splint and its solution, dispose of the burned splint in the 150 ml beaker containing water. 8. Observe and carefully describe the color of the flame on the data table. For example, describe the color as pinkish red or violet red instead of just red. 9. Repeat for the remaining solutions including the unknown solution. Be as descriptive and accurate as possible. 10. Identify the metal in your unknown solution. Clean-Up: Rinse everything well with tap water followed by a quick DI water rinse. Clean your benchtop. Put all equipment back exactly where you found it. GCC CHM 151LL: Atomic Theory GCC, 2017 page 3 of 8
Name: Atomic Theory Pre-lab assignment Purpose: Summary of procedure: Drawing of apparatus used: Instructor signature: Pre-lab points: Date: GCC CHM 151LL: Atomic Theory GCC, 2017 page 4 of 8
Name: Atomic Theory Pre-Lab Questions and Calculations 1. (1 pt) How are wavelength and energy related? Directly or Inversely 2. (1 pt) Calculate the frequency of red light with a wavelength of 705 nm? 3. (1 pt) Calculate the energy, in J, of violet let with a frequency of 6.99 x 10 14 Hz. 3. Absorption And Emission Spectra: A Web Tutorial This activity of the lab involves an interactive tutorial to help explain the process electrons go through when absorption and emission spectra are obtained from pure substances. Go to the Website link http://www.wwnorton.com/college/chemistry/chemistry3/ch/07/chemtours.aspx Click on Light Emission and Absorption only for the correct video tutorial. a. (1 pt) Describe what happens to an electron when it absorbs energy. Describe the location of the electrons in terms of energy levels and distance from the nucleus. b. (1 pt) Describe what happens when an electron in an excited atom returns to its ground state. GCC CHM 151LL: Atomic Theory GCC, 2017 page 5 of 8
Name: Partners: Data: Atomic Theory Lab Report Data Table 1: Spectrophotometric Analysis of Visible Light Shade in all colors between the wavelengths (in nm) listed below: 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 Data Table II: Gas Charge Demonstration Sample 1 Gas: Color of lamp: Spectral Lines 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 Sample 2 Gas: Color of lamp: Spectral Lines 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 Data Table III: Flame Test Solution Observation Unknown # Results: GCC CHM 151LL: Atomic Theory GCC, 2017 page 6 of 8
Results Table 1: Flame Test Results Unknown # Solution Evidence Discussion Questions: 1. (4 pts) Determine if energy is absorbed or emitted for each electron transition in a hydrogen atom: a. from n = 4 to n = 2 emitted absorbed b. from n = 2 to n = 6 emitted absorbed 2. (4 pts) You are the student representative for Glendale Christmas fireworks display. Explain which chemicals should be used in the fireworks to have the fireworks match the traditional Christmas decorations. (Hint: Consider your flame test results.) 3. (4 pts) KSLX FM radio rocks out at 100.7 MHz (FM radio station frequencies are in megahertz). Calculate the wavelength in meters for this Phoenix station. 4. (4 pts) Which travels faster X rays or microwaves? Explain your answer. 5. (4 pts) The emission spectrum of hydrogen gives four distinct wavelengths representing four colors of light. The tutorial lists the colors as: violet (410nm), blue (434nm), green (486nm), and red (656nm). Which color of light in the visible emissions spectrum for hydrogen has the highest energy photons? Explain. 6. (10 pts) Several Predators (aliens that see IR radiation) have landed near your house one dark night, and Arnold is nowhere to be found. Your filtered lamp is emitting radiation of 5.45 x 10-19 J per photon of energy. If Predators can only sense IR radiation, will they see the radiation from your lamp? Calculate the frequency and wavelength of the lamp emissions and GCC CHM 151LL: Atomic Theory GCC, 2017 page 7 of 8
then look at section 3.1, figure 2 in the Openstax Chemistry: Atoms First online book to determine if this radiation is in the IR range. = Hz = nm Type of radiation Conclusion: Summarize the results for determining the identity of the unknown solution from the flame test. Use data to support. Discuss at least two sources of error and how they can be corrected in the future. Also discuss the results from the demonstration and the Spec 20 color analysis. GCC CHM 151LL: Atomic Theory GCC, 2017 page 8 of 8